Step 1: how much roof
A DOE Office of Indian Energy course prepared with NREL gives power densities for crystalline silicon systems: 11 watts DC per square foot for a flush-mounted sloped roof and 8 watts DC per square foot for a flat roof. The same course notes that panel efficiency determines the footprint, not the energy production, and that the footprint varies by technology, tilt and location. These are planning averages, not a panel spec. Divide watts by density to get area.
At 11 watts per square foot, one kilowatt (1,000 watts) needs about 91 square feet of sloped roof, and a 7.7 kW system needs about 700 square feet. On a flat roof at 8 watts per square foot, one kilowatt needs about 125 square feet, because panels are tilted and spaced to avoid shading each other. Higher-efficiency panels fit more watts in the same area, so the real number for a given panel model is better read from its datasheet.
The 7.7 kW example is not arbitrary. Berkeley Lab's 2026 distributed solar data update reports a median residential system size of 7.7 kW DC for 2025 installs in the United States, which is the size this site's payback calculator starts from.
Usable area is smaller than total area
Not every square foot of a roof can carry panels. Obstructions and shaded sections of a roof reduce the area that can carry panels. An installer measures what fits; a model cannot. For a first pass, count only the roof planes that face south, east or west and are not shaded for most of the day.
Step 2: how much it produces
Production per kW depends on the sun and the setup, not on how large your roof is. This site models each Texas place with the PVWatts v8 calculator for a 1 kW array: fixed roof mount, 20 degree tilt, due south, standard modules, 14 percent system losses and typical-year weather. Across the 1220 Texas places modeled, the median is 1,515 kWh per kW DC per year. The range runs from 1,325 kWh in the weakest place to 1,883 in the strongest, which is about 42 percent between the two.
| Place | kWh per kW per year | 6 kW system, kWh per year |
|---|---|---|
| Houston | 1,422 | 8,532 |
| Dallas | 1,500 | 9,000 |
| Austin | 1,483 | 8,898 |
| San Antonio | 1,505 | 9,030 |
| Amarillo | 1,720 | 10,320 |
| El Paso | 1,835 | 11,010 |
The table is arithmetic on the model: output times system size. The city pages in the state hub and the solar index list every modeled place with a monthly breakdown. In this model El Paso and Amarillo come out well above Houston.
Step 3: put them together
Take the median Texas figure and the typical system size. A 7.7 kW system at 1,515 kWh per kW produces about 11,666 kWh a year in the model, or roughly 972 kWh a month on average. The EIA reports average Texas residential use of about 1,110 kWh a month, so a system that size covers a large share of an average home's use in total energy terms, but production peaks in summer afternoons and your use may not match that timing.
What the electricity is worth depends on how much you use as it is made and what your provider pays for the rest. The buyback guide explains that, and the calculator lets you price it under different plan structures.
Three system sizes side by side
| System size | Sloped roof at 11 W per sq ft | Flat roof at 8 W per sq ft | Modeled output at the Texas median |
|---|---|---|---|
| 4 kW | about 364 sq ft | about 500 sq ft | 6,060 kWh a year |
| 7.7 kW | about 700 sq ft | about 962 sq ft | 11,666 kWh a year |
| 10 kW | about 909 sq ft | about 1,250 sq ft | 15,150 kWh a year |
The area columns divide watts by the two DOE densities. The output column multiplies system size by the Texas median of 1,515 kWh per kW. Output scales with size and does not depend on the roof type in this model, which assumes the same 20 degree tilt for both.
Sizing to your own use
Work backward from your bills. An average Texas home used about 1,110 kWh a month in 2025 according to the EIA, which is 13,320 kWh a year. Dividing by the Texas median of 1,515 kWh per kW gives about 8.8 kW of panels to match that annual use in the model, and at 11 watts per square foot that is roughly 799 square feet of sloped roof. Your numbers will differ: put in your own twelve-month total from your bills and the output for your city from its page.
Matching annual use is a starting point and not a recommendation. Whether a system that size is sensible depends on its cost, on what your provider pays for the electricity you send to the grid, and on how much of the output you use as it is made. A smaller system that you use fully can pay back differently from a larger one whose surplus is bought at a low rate.
What moves the numbers
- Direction. The model assumes due south. East- and west-facing planes produce less in total, though west-facing ones shift output later in the day.
- Tilt. The model uses 20 degrees. Your roof pitch may differ.
- Shade. The model has none, so any shade on your roof lowers the real figure.
- Losses. The 14 percent default covers wiring, inverter and soiling. Real losses differ.
- Weather year. The model uses a typical year; any actual year can be higher or lower.
Questions
Is more roof always better?
More roof allows a bigger system, but the right size depends on your electricity use, the budget and the buyback rate on the excess, not on the roof alone.
Can I use the table instead of a quote?
No. It is a regional model for planning. An installer's site visit measures your roof, shade and equipment.
This page is general information, not advice for your situation.